1. Field of the Invention
This invention relates to a multi-point frame-fixing unit type linear displacement measuring apparatus formed so that a frame thereof containing a main scale therein and extending in the length measuring direction is fixed to an object machine at a multiplicity of points of not smaller than three points, and a method of fixing the apparatus.
2. Description of the Related Art
In order to detect the linear displacement of a relatively moving object machine (moving body) for the purpose of carrying out, for example, a feedback control operation in a machine tool and an industrial machine, a unit type linear scale has been generally used, which is formed in one body as shown in FIG. 1, by an elongated frame 10 of, for example, aluminum in which a main scale of, for example, glass is housed, and a detecting head 14 relatively movable with respect to the frame 10 and having a slider connected thereto and including an index scale for scanning the main scale.
The unit type linear scales include a so-called a multi-point frame-fixing unit type linear scale in which the aluminum frame 10 is fixed directly to an object machine by, for example, fixing screws as shown in FIG. 1.
In this multi-point frame-fixing unit type linear scale, the fixing of the scale is done at a multiplicity of points of not smaller than three points (for example, five points) with the frame 10 contacting a surface of an object machine (an illustration is omitted), so that this scale has the advantages of resisting an external force, such as vibration and an impact. This scale is also advantageous in that, when the number of the scale-fixing positions is increased by arranging the fixing screws 12 at constant intervals of, for example, 200 to 250 mm through the frame 10 over the whole length thereof, the dependency of the resonance frequency of the frame 10 upon the length thereof can be reduced to a low level.
However, when a method of fixing the scale to a machine tool formed mainly out of iron, by completely securing the scale at all of the mentioned points by the fixing screws 12 is used, thermal stress caused by temperature variation at the screw-fixed portions occurs due to a difference between the linear expansion coefficient of iron and that of aluminum. This thermal stress causes the deformation and distortion of the frame 10 and the main scale provided in the interior thereof, and the accuracy of the scale to be deteriorated. In addition, due to the thermal stress as well occurring in the screw fixed portions, the position of the frame 10 with respect to the surface of the object machine slips minutely, and the origin stability of the position of the scale in the length measuring direction is deteriorated.
The present invention has been made so as to solve these problems of the related art apparatus and method of this kind, and mainly aims at minimizing the occurrence of thermal stress with excellent characteristics with respect to an external force, such as vibration and an impact maintained.
The present invention solves the above-mentioned problems of a multi-point frame-fixing unit type linear displacement measuring apparatus which is formed such that a frame thereof containing a main scale therein and extending in the length measuring direction is fixed to an object machine at a multiplicity of points of not smaller than three points, by fixing some portions in the length measuring direction of the apparatus completely to an object machine, and some other portions in the same direction thereof elastically to the object machine by using bushes provided with an elastic layer between the bushes and the frame.
The first-mentioned parts of the apparatus are fixed to the object machine completely by using a plurality of fixing members.
The thickness of the elastic layer of such bushes may be set larger as the layer is far from the completely fixed portions is set larger.
The present invention provides a method of fixing a multi-point frame-fixing unit type linear displacement measuring apparatus formed such that a frame thereof containing a main scale therein and extending in the length measuring direction is fixed to an object machine at a multiplicity of points of not smaller than three points, in which some portions in the length measuring direction of the frame are fixed completely to the object machine, some other portions in the length measuring direction thereof being fixed elastically to the object machine by using bushes provided with an elastic layer between the bushes and the frame.